U.S. patent number 4,861,329 [Application Number 07/280,714] was granted by the patent office on 1989-08-29 for centrifugal separator.
This patent grant is currently assigned to Alfa-Laval Separation AB. Invention is credited to Leonard Borgstrom, Claes G. Carlsson, Peter Franzen, Claes Inge, Torgny Lagerstedt, Hans Moberg, Olle Nabo.
United States Patent |
4,861,329 |
Borgstrom , et al. |
August 29, 1989 |
Centrifugal separator
Abstract
In the rotor of a centrifugal separator a stack of conical
separation discs (4) is arranged concentric with the rotor axis.
The rotor has an inlet for a dispersion and an outlet for liquid
having been freed from a substance dispersed therein. In each space
between adjacent separation discs (4) the disc surface from which
the dispersed substance moves away as a consequence of centrifugal
force during rotor operation has flow influencing members, whereas
the surface of the other separation disc, situated opposite to said
members, is substantially smooth. The relation (L/H) where L is the
distance between adjacent flow influencing members and H is the
distance between the separation discs, and the relation (1/H) where
1 is the extension of each flow influencing member along the disc
surface and H is the distance between the separation discs, are
larger than zero but less than 2.
Inventors: |
Borgstrom; Leonard (Bandhagen,
SE), Carlsson; Claes G. (Tullinge, SE),
Franzen; Peter (Tullinge, SE), Inge; Claes
(Saltsjo-Duvnas, SE), Lagerstedt; Torgny (Stockholm,
SE), Moberg; Hans (Stockholm, SE), Nabo;
Olle (Tullinge, SE) |
Assignee: |
Alfa-Laval Separation AB
(SE)
|
Family
ID: |
20370513 |
Appl.
No.: |
07/280,714 |
Filed: |
December 6, 1988 |
Foreign Application Priority Data
Current U.S.
Class: |
494/73 |
Current CPC
Class: |
B04B
7/14 (20130101); B04B 1/08 (20130101) |
Current International
Class: |
B04B
7/00 (20060101); B04B 1/00 (20060101); B04B
1/08 (20060101); B04B 7/14 (20060101); B04B
001/08 () |
Field of
Search: |
;494/68,69,70,71,72,73,74,75 ;210/781,782,360.1 ;422/72 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Jenkins; Robert W.
Attorney, Agent or Firm: Davis Hoxie Faithfull &
Hapgood
Claims
What we claim is:
1. In a centrifugal separator for the separation of a substance
dispersed in a liquid, comprising a rotor having a separation
chamber, an inlet for a dispersion, an outlet for separated liquid,
a stack of conical separation discs arranged coaxially with the
rotor in the separation chamber and having interspaces between
them, and flow influencing members situated in at least part of the
interspaces between the separation discs, which members in each of
said interspaces are arranged in contact with the surface of the
one of the separation discs, from which the dispersed substance
moves away during operation of the rotor as a consequence of
centrifugal force but at a distance from the other separation disc
towards which the dispersed substance moves during operation of the
rotor as a consequence of centrifugal force so that a space is
formed between the members and said other separation disc admitting
flow of dispersion in the circumferential direction of the rotor
past the members, said flow influencing members forming flow paths
between themselves extending between radially outer and inner areas
of said one separation disc, the improvement in which
said flow influencing members are formed and positioned to prevent
to a substantial degree the formation of Ekman layers along the
surface of said one separation disc and so that the relation (L/H),
where L is the distance between adjacent flow influencing members,
seen in the circumferential direction of the rotor, and H is the
distance between the surfaces of the separation discs, and the
relation (1/H), where 1 is the extension of each flow influencing
member in the circumferential direction of the rotor and H is the
distance between the surfaces of the separation discs, are greater
than zero but less than 2;
and the surface of said other separation discs, opposite to the
flow influencing members is formed to promote the formation of an
Ekman layer along the same during operation of the rotor.
2. Centrifugal separator according to claim 1 in which the flow
influencing members are formed and placed to give said one
separation disc a substantially homogeneous surface structure on at
least a part of one side of said disc.
3. Centrifugal separator according to claim 1 in which the flow
influencing members have the same shape.
4. Centrifugal separator according to claim 3 in which each flow
influencing member has substantially the same extension in all
directions along the surface of said one separation disc.
5. Centrifugal separator according to claim 1 wherein the relation
(h/H) is in the range from about 0.2 to about 0.5, h being the
height of the flow influencing member.
6. Centrifugal separator according to claim 1 wherein the relation
L/H, and the relation 1/H are larger than 0.2 but less than 1.0.
Description
The present invention relates to a centrifugal separator for the
separation of a substance that is dispersed in a liquid, comprising
a rotor having a separation chamber, an inlet for a dispersion, an
outlet for separated liquid, a stack of conical separation discs
arranged coaxially with the rotor in the separation chamber, and
flow influencing members situated in at least a part of the
interspaces between the separation discs. These members in each of
said interspaces are arranged in contact with the surface of the
one of the separation discs, from which the dispersed substance
will move away during operation of the rotor as a consequence of
centrifugal force, but at a distance from the surface of the other
separation disc towards which the dispersed substance will move
during operation of the rotor as a consequence of centrifugal
force, so that a space is formed between the members and said other
separation disc which admits flow of dispersion in the
circumferential direction of the rotor past the members, said flow
influencing members forming flow paths between themselves extending
between radially outer and inner areas of said one separation
disc.
A centrifugal separator of this kind, described in the Swedish
patent specification No. 7503054-4, is equipped with flow
influencing members in the form of radially extending ribs. It is
stated that these ribs give the result that in each interspace
between the separation discs "the flow is distributed in a manner
such that the largest part (80-90%) of the suspension flows in the
interspaces between the ribs 15", whereas in the space between the
ribs and the separation disc towards which suspended particles move
during the rotor operations as a consequence of centrifugal force,
"there are formed stagnation zones, where the suspension flows at a
small speed". As a consequence thereof, it is further stated, a
decrease of the speed gradient is obtained near the separation disc
surface towards which suspended particles are moved by the
centrifugal force, so that a more effective separation of these
particles may be obtained. The efficiency of a centrifugal
separator, it is said, may thereby be increased 2-5 times in
comparison with that of a conventional centrifugal separator.
Neither the details about the shape and location of the ribs nor
the explanation as to the function of the ribs given in said patent
specification can be used in practice to obtain an improvement of
the efficiency of a centrifugal separator, in accordance with what
is alleged. The reason therefore will be evident from the
following.
The object of the present invention is to provide a centrifugal
separator of the type described which is designed such that a
substantial improvement of the separation efficiency can be
obtained by means of flow influencing members between the
separation discs.
According to the invention this is possible if the flow influencing
members are formed so as to prevent, to a substantial degree, the
formation of so-called Ekman layers along the surface of said one
separation disc. This involves having the relation between the
distance between adjacent flow influencing members, seen in the
circumferential direction of the rotor, and the distance between
the separation disc surfaces, and also the relation between the
extension of each member in the circumferential direction of the
rotor and the distance between the separation disc surfaces, larger
than zero but less than 2, preferably between 0.2 and 1.0.
According to the invention the surface of said other separation
disc, opposite to the flow influencing members, is formed in a
manner known per se for the obtainment of Ekman layers along the
same during operation of the rotor.
By this invention it is possible to prevent formation of so-called
Ekman layers at the separation disc surfaces having flow
influencing members and, instead, to establish close to these
separation disc surfaces a flow providing the same effect as a very
thick hypothetical Ekman layer. In other words, the effect is
obtained that the radial flow of dispersion in each interspace is
distributed such that the main part of the radial flow arises close
to the flow influencing members and only a small part arises near
the separation disc surface towards and along which substance
separated from the dispersion should move. By the particular shape
of the flow influencing members, there is avoided a turbulent flow
of the dispersion in the interspace between the separation discs
which would counteract an effective separation of the dispersed
substance. Such an undesired turbulence between the separation
discs may arise in an arrangement of the flow influencing members
according to the previously mentioned Swedish patent specification.
Furthermore, in this known arrangement so-called Ekman layers will
be formed between the ribs as well as on the upper sides of the
same, for which reason the radial flow of dispersion will be of
substantially the same magnitude along both of the two separation
discs limiting the interspace in question.
In a preferred embodiment of the invention the flow influencing
members have the form of evenly distributed protuberances from said
one separation disc surface in each plate interspace, each
protuberance having substantially the same extension in all
directions along the disc surface. By such a rough and a
homogeneous structure of the disc surface substantially uniform
flow conditions can be obtained along the whole disc surface.
The invention is described in the following with reference to the
accompanying drawing, in which:
FIG. 1 is a view in vertical section of a centrifugal separator
having conical separation discs, to which the invention is
applicable.
FIG. 2 is a plan view of a conical separation disc;
FIG. 3 is a schematic view in radial section through two smooth
separation discs and the interspace therebetween;
FIG. 4 is a schematic plan view of part of a separation disc
provided with flow influencing members according to the invention;
and
FIG. 5 is a schematic fragmentary view in vertical section showing
an interspace similar to that in FIG. 3 but where one of the
separation discs has flow influencing members according to the
invention.
FIG. 1 shows a centrifuge rotor 1 supported by a vertical drive
shaft 2. Within the rotor a separation chamber 3 is formed in which
is arranged, coaxially with the rotor, a stack of frustoconical
separation discs 4. The rotor 1 has a central inlet chamber 5 for a
dispersion of components to be separated in the separation chamber
3, and a central outlet chamber 6 for a separated relatively light
liquid. A stationary inlet tube 7 extends into the inlet chamber 5,
and stationary outlet member 8 extends into the outlet chamber 6.
At its periphery the rotor has an intermittently openable outlet 9
for a separated relatively heavy component, e.g., sludge, that
before the separation constituted the dispersed phase of the
supplied dispersion. The inlet chamber 5 communicates with the
separation chamber 3 through several radial channels 10 evenly
distributed around the rotor axis. Through an overflow outlet 11
the separation chamber 3 communicates with the outlet chamber
6.
FIG. 2 shows a separation disc 4 which on its upper side is
provided with a number of radially extending ribs 12 intended to
serve as spacing means between this separation disc and an adjacent
separation disc in a centrifuge rotor according to FIG. 1. The
intended direction of rotation is shown by means of an arrow R.
During operation of a centrifuge rotor according to FIG. 1 a
dispersion supplied to the inlet chamber 5 is caused to rotate at
the same speed as the rotor during its passage through the radial
channels 10. The angular speed which the dispersion has reached in
the area of the outer edges of the separation discs 4 will increase
further when the dispersion is forced to flow back towards the
rotor axis between the separation discs. This increase of the
angular speed, depending on the fact that each part of the rotating
dispersion is striving to maintain its momentum, cannot be
prevented by spacing members between the separation discs, such as
ribs of the kind shown in FIG. 2.
As a consequence of the above a flow of dispersion will arise in
each interspace between adjacent separation discs, that is directed
substantially around the rotor axis. This flow having a speed in
the circumferential direction of the rotor larger than that of the
separation discs themselves is named, in the description which
follows, a geostrophic flow. A flow line for part of this
geostrophic flow is shown in FIG. 2 and is designated 13. As shown,
the ribs 12 form obstacles to a substantially circular geostrophic
flow. Such a circular flow can be obtained, however, if the ribs
are substituted by spot-like protuberances as sometimes used.
The geostrophic flow of the dispersion moving around the rotor
axis, i.e., substantially in the circumferential direction of the
rotor, the formation of which, however, thus depends on the fact
that the dispersion is forced to move towards the rotor center
through the disc interspaces, meets friction at the surfaces of the
separation discs. As a consequence of this friction a flow of
liquid arises in a very thin layer closest to each disc surface,
which flow has a substantially larger component directed radially
inwards than the geostrophic flow, at least where the latter goes
in the circumferential direction of the rotor. The thin layer is
usually called an Ekman layer. In the case just described, when the
geostrophic flow moves faster than the separation discs, the liquid
in the Ekman layers flows along the disc surfaces radially inwards
If the geostrophic flow had been moving slower than the separation
discs, which would have happened if the dispersion had been forced
to move radially outwards through the disc interspaces, the liquid
in the Ekman layers would instead have been flowing radially
outwards
FIG. 3 illustrates how the radial flow may be distributed in
different layers of an interspace between two conventional smooth
separation discs 4a and 4b. The rotor axis is illustrated by a line
2a. The radial flow velocity is zero at the surfaces of the
separation discs and substantially zero also in a large area 14
midway between the separation discs. A substantial radial flow
exists only in two layers 15 and 16 close to the separation discs.
These layers are the two so-called Ekman layers. Substantially all
dispersion to flow through the space between the separation discs
4a and 4b from their outer edges to their inner edges is thus
forced to flow radially inwards in the layers 15 and 16 The
thickness of each Ekman layer for most practical operating
conditions is on the order of 1/10 of the distance between two
adjacent separation discs.
A substance dispersed in the dispersion, e.g., small solids heavier
than the carrying liquid, will, under the centrifugal force in the
interspace between the separation discs, strive to move radially
outwards towards the separation disc 4a and along this towards its
outer edge. Such a flow of solids towards and along the separation
disc 4a will be made difficult by the radially directed dispersion
flow in the layer 15. Therefore, it would be desirable to
accomplish, if possible, a different distribution of the radially
inwards directed flow of the dispersion, so that it would be
smaller in the area 15 and larger in the area 16. Such a desired
flow distribution is shown by a dotted line in FIG. 3.
According to the invention this can be accomplished by providing
the separation discs 4 on their upper sides with flow influencing
members 17 shaped in a particular way, such as can be seen from
FIG. 4 and FIG. 5. The flow influencing members 17 have to be so
formed that they give the upper side of each separation disc a
rough surface structure, which prevents the formation of an Ekman
layer thereon. Furthermore, they have to be formed so that even if
they create a substantially larger friction resistance for the
geostrophic flow along said upper side than a smooth surface would,
they will still not cause turbulence in a large part of the disc
interspace. Such turbulence would make it difficult or impossible,
for the intended separation of the dispersed substance to take
place. According to the invention the flow influencing members, in
order to obtain the desired effect, have to be formed so that the
relation between the distance between adjacent members, seen in the
circumferential direction of the rotor, and the distance between
the separation discs, and also the relation between the extension
of each member in the circumferential direction of the rotor and
the distance between the separation discs, is less than 2.
The just used expression "in the circumferential direction of the
rotor" should be understood as "in the direction of the geostrophic
flow".
Flow influencing members may not be required across the whole upper
side of each separation disc. Particularly if ribs or other flow
obstacles are present in the plate interspaces it is possible that
flow influencing members may be avoided over parts of said upper
side.
FIG. 5 shows a section through parts of two adjacent separation
discs 4c and 4d and the interspace therebetween. The upper side of
the lower disc 4d has a number of flow influencing members 17 (see
also FIG. 4) each with an extension 1 along the plate surface and a
height h above the same. The distance between two adjacent flow
influencing members is designated L and the distance between the
separation discs is designated H. The direction of the geostrophic
flow in the disc interspace is shown by an arrow G.
Generally accepted theories about so-called Ekman layers show that
formation of an Ekman layer requires a geostrophic flow a certain
minimum distance along a surface. This distance is relatively
short. By the above defined relation between the distance between
the separation discs and the mutual distance between the flow
influencing members and their extension along the disc surface in
question, respectively, i.e., that 1/H and L/H should be less than
2, no Ekman layer will be formed on the upper side of the
separation disc 4d in connection with practically used parameters
such as flow, viscosity, rotational speed, etc., for centrifugal
separators of the kind here concerned. Furthermore, by the defined
relation turbulence in the disc interspace above the flow
influencing members 17 is avoided.
The height h of each flow influencing member 17 may vary within
wide limits according to the invention. Preferably, however, the
relation h/H, i.e., the relation between the height of each member
and the distance between the separation discs, should be in the
range 0.2-0.5.
In a centrifugal separator of the kind for which the invention is
intended, the disc plate thickness usually is on the order of
0.5-1.0 mm, and the distance (H) between adjacent discs is on the
order of 0.5-1.5 mm. This means that flow influencing members
formed according to the invention may have a height of, for
instance, 0.1-0.7 mm and an extension along the separation disc
surface and the geostrophic flow of for instance 0.2-3.0 mm.
The invention has been described above applied to a case where a
dispersion contains a dispersed substance heavier than the
continuous phase of the dispersion. The invention can also be used
in connection with separation of a dispersed substance which is
lighter than the continuous phase of the dispersion, e.g.,
separation of cream from milk.
In this case the flow influencing members should be situated on the
under side of the conical separation discs, i.e., on the disc side
from which the dispersed substance moves away owing to centrifugal
force during operation of the rotor.
As mentioned above the upper or lower sides of the separation discs
need not be covered entirely by flow influencing members. Depending
upon the shape of necessary spacing means between the separation
discs, varying directions of geostrophic flow may arise. Flow
influencing members are most important in the part of a disc
interspace where the strongest counter-flow can be expected between
the separated dispersed substance an Ekman layer formed as a
consequence of the geostrophic flow.
Only one form of the flow influencing members has been described
above. Any other form thereof is possible within the scope of the
invention which will give parts of the separation discs a rough
surface structure. A rough surface structure may be difficult or
expensive to accomplish on separation discs made of metal.
Therefore, the invention may prove to be especially applicable, in
practice, when the separation discs are made of plastic, the flow
influencing members being made in one piece with the separation
discs.
* * * * *